The United Kingdom's attempt to claim 200,000 square kilometres of the south Atlantic seabed around Ascension Island for mineral and oil exploration has been unexpectedly dismissed by a United Nations panel of experts.

The Commission on the Limits of the Continental Shelf (CLCS) concludes that the volcanic pinnacle on which Ascension rests is too slender to generate rights to an extended zone of the submerged continental shelf. The island is an overseas British territory.

The UN convention on the Law of the Sea allows states to expand sovereignty across the ocean floor beyond the traditional 200-mile limit if they can demonstrate the "prolongation" of an adjoining continental shelf.

Proof of eligibility depends on various formulae, including tracing the 2,500m submarine contour, establishing the floor of the continental shelf and measuring sedimentary thickness.

If successful, countries may be granted the right to extract oil, gas and minerals from the seabed up to, and sometimes more than, 350 miles beyond their coastlines.

The waters surrounding Ascension are so deep that exploitation probably depends on engineering innovations and was being promoted as an investment.

The Foreign Office confirmed the application had been turned down but made no formal comment. The UK is likely to appeal against the CLCS ruling on the grounds that the regulations have not been correctly interpreted.

Such claims have been dismissed by environmental groups as "colonial land grabs" and have triggered fierce international disputes.

Britain's other Atlantic seabed claims include the depths around Rockall in the north Atlantic and the icy waters surrounding the Falklands and South Georgia in the south Atlantic.

Both of these applications have been frozen by diplomatic protests. The Faroe Islands oppose the Rockall submission while Argentina and Chile object to the south Atlantic claim.

The Foreign Office believed the Ascension application was the most simple because there were no counter-claims from neighbouring states.

In its 21-page judgment, the CLCS ruled that: "[Ascension Island] is not connected to any other discrete morphological feature that rises above the general 'ruggedness' of the surrounding seafloor."